1. Technical Field of the Invention
The present invention relates to an image forming apparatus for use in copying machines and various printers which are provided with electrophotographic system, and more particularly, to an improvement of the control of surface potential of a photoconductor used for image formation.
2. Description of Related Art
Heretofore, various apparatuses have been offered as a mode of charging means for a photoconductor in which high voltage is applied to a charge wire which is arranged opposite to the photoconductor to generate corona discharge in order to provide the surface of the photoconductor with uniform charge thereon.
However, in case of a photoconductor which encounters great light fatigue and possesses attenuation characteristic such as a photoconductor in which selenium is used as a photoconductive member, even if a fixed charge is applied in the process of electrification, the charge maintaining capability is lowered according to the degree of light fatigue so that the charged potential is not made uniform in the area of development.
For instance, Japanese Patent Publication TOKKO SH059-37500 discloses a control device to cope with the problem. The device is arranged to stabilize the charge maintaining capability at a low value by prolonging a period of exposure corresponding to the time an image is left unformed in which the photoconductor is uniformly charged prior to image forming process. However, since a preliminary process is conducted for correcting the sensitivity of the photoconductor after print switch is turned on at the time when image forming operation is performed, there arise problems that the time required for forming a first image can not be fixed, and moreover, it takes a very long time for the image forming process of the first image after the photoconductor is left unused for a long period of time.
When the inventor of the present invention has carefully examined light fatigue characteristic of the photoconductor, it became clear that there occurs a kind of fatigue characteristic composed of a fatigue characteristic which progresses to a stablized state in a short time and a fatigue characteristic which gradually progresses to a stabilized state in a long period of time.
The reason for producing such light fatigue characteristic can not be made clear fully, however, the following reasons may be considered.
The photoconductor is arranged to receive light through exposure, eraser, main eraser and the like, while the eraser is generally constructed by arranging a plurality of LED so as to correspond with changes in the size of transfer sheet and magnification. In the wave length of light output from the LED, a wave length which does not contain long wave length is preferable since the light fatigue and a drop of charged potential are less. In an experiment conducted, it is confirmed that in case when a long wave length of light is received, the amount of drop in charged potential is large and the charged potential is kept falling for over a long period of time thus taking extremely long time until it is stabilized as against the case when a short wave length of light is received wherein the amount of drop in charged potential is comparatively smaller and the charged potential is stabilized at a fixed value in a short time. However, in the LED which outputs only a short wave length of light (green light), sufficient luminous amount can not be obtained, and therefore, the LED which outputs a long wave of light (yellow light) is practically utilized sacrificing the light fatigue to some extent.
In the photoconductor in which selenium is used, as shown in FIG. 8, when photoconductive member S is made vacuum evaporation to a conductive base plate B, a multiplicity of traps t, free radicals among atoms, are formed, and charge carriers c generated by irradiation of light are captured by the traps t and is sprung out to reach the conductive base plate B. When there are the more traps t, the more time is required for the charge carriers c to reach the conductive base plate B, and the mobility of the charge carrier c is reduced, which means dark decay is less. However, once a copying operation is started, the number of traps t appears to have been decreased since the traps t are filled by the charge carrier c, and the mobility of the charge carrier c becomes markedly high. Consequently, dark decay becomes large and the surface potential is lowered since the charge maintained on the surface of the photoconductor is neutralized even by a small amount of light. This is considered as a cause of rapid light fatigue at an initial stage.
On the other hand, the photoconductor which uses selenium is arranged in monolayer (single layer), and attainable depth of the charge carriers differs depending on the wave length of light. A short wave length of light L.sub.1 is absorbed at a shallow area and generates the charge carriers c, while a long wave length of light L.sub.2 is absorbed in deep layer and generates the charge carriers c. Since the selenium system photoconductor is P-type semiconductor, positive charge carriers c are able to easily move to the conductive base plate B through the charged electric field E, however, since the range of movement of negative charge carriers c is so small that the charge carriers c generated by the long wave length of light L.sub.2 in the deep layer is not able to easily reach the surface. Consequently, the charge carriers c are gradually accumulated, and electric charge on the surface of the photoconductor is neutralized since the charge carrier c act as negative charge, thereby showing a phenomenon that the surface potential appears to have been lowered. This phenomenon is considered as a cause of light fatigue which is increased in a long period of time. It is thus considered that the rapid light fatigue at an initial stage and the gradually increasing light fatigue in a long period of time compose the light fatigue characteristic as described above.